The present invention relates to resins formed from oligomers bearing chemically functional terminal groups. The backbones of the oligomers are polysulfones or polyether sulfones, and the functional groups have phenolic configurations. The functional groups are chemically attached to the backbones by imide moieties.
Composite materials are finding ever increasing utility in the structural engineering art of the aerospace industry. Emphasis is being placed on developing new composites and improved methods and resin systems for binding and holding the composite reinforcing materials together.
Although the most popular resins for composite matrices are epoxy resins, they suffer from two major deficiencies. First is their propensity to absorb moisture, which is deleterious to the mechanical properties of high performance structural composites. Property degradation may constitute an initial reversible degradation resulting from plasticization of the epoxy resin matrix by absorbed moisture or may constitute permanent degradation resulting from cracks in the epoxy resin matrix. The initial degradation can be predicted in neat epoxy resin castings based on standard diffusion models. Permanent degradation in composites cannot be predicted in any straightforward manner, however, and is influenced by thermal peaks during exposure. Consequently, there has been a need to provide resin systems for composite structures that do not suffer from moisture degradation.
The second major deficiency with epoxy resins is their brittle nature. The inherent brittleness of epoxy systems makes them highly susceptible to damage, causing aerospace designers to forego the potential weight savings of the composite system to achieve reliability. Although there are current systems that are damage tolerant (tough), they are thermoplastic in nature. The current systems that have the required toughness are the polysulfone and polyether sulfone systems.
Reinforced phenol-formaldehyde composites and laminates have long been in use. Such composites and laminates generally are not tough and are suitable only when high performance characteristics are unnecessary. Because such products have little or no elasticity, they fail to exhibit the impact resistance and crack propagation resistance needed for such high performance areas as in aircraft and space vehicles.
One attempt to improve the properties of available resin compositions is disclosed in U.S. Pat. No. 4,026,871 to D'Alelio. This patent discloses fitting phenolic groups to the ends of polymer chains that are fashioned with aromatic backbones. Unfortunately, the oligomers proposed by D'Alelio are not soluble in solvents used for making varnishes and therefore suffer from serious processing difficulties. The absence of suitable laminating varnishes has remained a serious problem in the fabrication of high performance, fiber-reinforced materials.